Known in the art are a composition and a method for preventing evaporation losses of petroleum and petroleum products by means of a floating covering of microcapsules (cf. U.S. Pat. No. 2,023,308 Cl.220-26; Oil and Gas J., 1953, v. 52 No. 10, p. 74-75; Oil and Gas J., 1955, v. 54 No. 23, pp. 248-251; Petroleum Times, 1960, v. 67, No. 1652, p. 824). The protective covering comprises a bulk material consisting of hollow microspheres with a diameter of 6.times.10.sup.-6 to 9.times.10.sup.-5 m filled with an inert gas. The thickness of the covering is equal to 25.times.10.sup.-3 m. U.S. Pat. No. 3,146,060 points out that covering of the surface of gasoline with microspheres according to U.S. Pat. No. 2,023,308 does not reduce, but on the contrary, intensifies the process of evaporation of gasoline due to the capillary effects as compared to evaporation of gasoline without such a covering. The above-specified U.S. Pat. No. 3,146,060 teaches a composition and a method for protection of the surface of petroleum products, wherein for the minimization of the capillary effects in the covering the microspheres should be coated with surface-active substances or surfactants such as silicon or fluorocarbon. This treatment is intended for preventing the microspheres from wetting and thus for reducing the capillary effects. However, this technique has proved to be less effective. After placing the microspheres onto the surface of a crude oil during the first few days the efficiency of protection from evaporation did not exceed 60%. This composition turned to be unsuitable for the protection of light-weight petroleum products from evaporation (cf. Chemical Engineering, 1960, v. 67, No. 25, p. 88). Also noted are other disadvantages of this composition. For example, microspheres are admitted into a tank together with the petroleum stream and then after intermixing with petroleum they ascend forming a layer of non-uniform thickness. Upon pumping petroleum out of the tank some portion of the microspheres is withdrawan from the tank and another portion sinks to the bottom. As a result, separate areas, not covered with the microspheres or covered with a layer of inadequate thickness appear on the surface of petroleum. This hinders operation of petroleum reservoirs and makes it rather more expensive in the storage of petroleum products. In this connection, this mode of protection of the surface of petroleum products from evaporation has not found extensive use in the art.
Currently employed is the protection of petroleum and petroleum products from evaporation by means of pontoons floating on the surface of petroleum products or floating roofs. A sealing unit is provided between the reservoir walls and the pontoon or the floating roof over the periphery thereof so that the area of evaporation of petroleum products is reduced to the minimum possible value. In the world practice up to 200 different types and sizes of such sealing units are known which can be classified as hard and soft ones.
A hard (mechanical) sealing unit consists of lever-type metallic members; soft (flexible) sealing units are made of a rubberized fabric or a polyurethane foam. It should be noted that in the case of using soft sealing units a strict monitoring of the pressure of the stored liquid medium is required since without it no reliable sealing of the unit can be ensured. Operation of such sealing units is rather complicated. Hard-type sealing units are neither sufficiently reliable and call for frequent inspection. Some parts of the sealing unit undergo corrosion and must be replaced with new ones from time to time. In addition to the above-mentioned disadvantages of mechanical means for protection of the liquid surface from evaporation still other disadvantages should be noted, namely: the use of expensive materials (metals, gasoline-resistant rubber), high labour expenditures, strict requirements to the design of the sealing unit, its manufacture, assembling and use and ah high production cost of such units.
The operation or pontoons shows that deviation of a reservoir from cylindricity along its axis results in a substantial decrease of the protective function of the pontoon. Therefore, none of the prior art protective means meets the requirements imposed on them in storage of petroleum products.